Ash could develop his character in Sinnoh by becoming more strategic in his battles. Since Sinnoh has a variety of strong and unique Pokemon, he'd need to think more carefully. Misty could develop by becoming more independent. Maybe she takes on some solo adventures in Sinnoh, away from Ash for a bit, and this helps her grow as a trainer.
In a fanfic about Pokemon working in Sinnoh with skill, trainers could play a crucial role. They might train their Pokemon to perform specialized tasks. The story could revolve around a trainer who discovers new ways to utilize their Pokemon's skills in Sinnoh. Maybe a Pikachu using its electric skills to power up machinery in an old mine in Sinnoh.
It's possible that he received it as a gift from Professor Oak. Oak might have discovered a way to transfer Mega Stones across regions and sent the Charizardite to Ash in Sinnoh. Another option could be that Ash made a trade with another trainer. He traded one of his Pokémon for the Mega Stone, knowing the potential of Mega Charizard.
Ash may develop his skills further in Sinnoh fanfictions. He could become more strategic in battles, learning from the different types of Pokémon and trainers in Sinnoh. May, on the other hand, might expand her knowledge of Pokémon contests, incorporating Sinnoh - style elements into her performances. Their experiences in Sinnoh help them grow both individually and as friends.
Well, in a fanfic, a Pokemon like Bulbasaur could use its grass - type skills to help with gardening in Sinnoh. It could plant and grow all kinds of beautiful flowers and useful plants.
In Pokemon fanfiction, the Love Stone can be depicted in various ways. Sometimes it's a rare item that has the power to make Pokemon feel affectionate towards their trainers. It might be used in a story where a trainer is trying to bond with a particularly difficult Pokemon.
In some fanfictions, Ash and May might get involved in a mystery in Sinnoh. For example, strange events are occurring that are affecting Pokémon behavior. They work together to solve the mystery, with Ash using his battle skills and May her knowledge of Pokémon contests. As they dig deeper, they find that an ancient power from Sinnoh's past is being misused.
Well, Thanos getting the Soul Stone in the comics is a convoluted process. There are specific storylines and encounters that lead to him obtaining it, but it's a wild ride full of surprises and challenges.